Numerous diagnostic and therapeutic procedures have been developed in which a catheter is transluminally advanced within a guide sheath or over a guidewire into various locations of a patient, such as the heart. These procedures conventionally have been conducted using X-ray and/or ultrasound imaging technology to facilitate guidance of the catheter through the body and to the target location. Unfortunately, X-ray imaging technology has a number of limitations, including limited anatomical visualization of the body and blood vessels, limited ability to obtain a cross-sectional view of a target vessel, and exposure of the subject to potentially damaging x-ray radiation.
Magnetic Resonance Imaging (MRI) technology has the potential to overcome these deficiencies. MRI has several distinct advantages over X-ray imaging technology, such as excellent soft-tissue contrast, the ability to define any tomographic plane, and the absence of ionizing radiation exposure. In addition, MRI offers several specific advantages that make it especially well suited for guiding various devices used in diagnostic and therapeutic procedures including: 1) real-time interactive imaging, 2) direct visualization of critical anatomic landmarks, 3) direct high resolution imaging, 4) visualization of a device-tissue interface, 5) the ability to actively track device position in three-dimensional space, and 6) elimination of radiation exposure.
Induced RF currents (referred to as RF coupling) on coaxial cables, electrical leads, guide wires, and other elongated devices utilized in MRI environments can be problematic. Such RE coupling may cause significant image artifacts, and may induce undesired heating and cause local tissue damage. To reduce the risk of tissue damage, it is desirable to reduce or prevent patient contact with cables and other conductive devices in an MRI environment. Such contact, however, may be unavoidable in some cases. For devices that are inserted inside the body, such as endorectal, esophageal, and intravascular devices, the risk of tissue damage may increase.
Various ways of limiting RF coupling have been proposed. For example, U.S. Pat. No. 7,215,121 describes a balun arrangement for use with an MR apparatus. U.S. Pat. No. 6,284,971 describes a coaxial cable adapted to resist undesired heating due to induced RF currents. U.S. Pat. No. 4,859,950 describes a balun circuit arrangement for RF coils in MR systems which addresses the adverse effects of induced currents in the cable system used for coupling the MR coils to the RF power transmitting and receiving equipment of the system. However, there remains a need for improved ways of reducing RF coupling in MRI environments.